The present invention relates to the technical field of fertilizers in general and more especially fertilizer compounds in the form of granules and their manufacturing process.
Fertilizers are organic or mineral plant nutrients intended to increase or maintain the fertility of plant life in particular by providing them with plant food elements such as nitrogen, potassium, phosphorous, calcium, magnesium, sulphur etc. as well as trace elements.
Conventionally, in order to facilitate their use and preservation, fertilizers are often produced in granular form.
In most cases, the process used to obtain granular fertilizers uses agglomeration without compression, referred to as wet granulation. This granulation technique is intended to convert materials that are, generally speaking, powdery into small pellets.
Agglomeration is obtained by feeding into an agitator (of the drum granulator or plate granulator type) firstly the powdery powder (or a mixture of solids) and secondly a small quantity of a liquid which is generally water. The effect of this is to cause the particles to stick together, thus forming agglomerates which assume a spherical shape as they roll and rub against each other.
Of the numerous factors that characterise the granules and the effectiveness of the granulation operation, the main factors are the size of the particles and the size distribution as well as the hardness of the granules obtained.
Conventionally, various additives are often used, including binders that are intended to reduce the quantity of water used and hence the cost of drying.
The main binders used to produce granular fertilizers include:                sulphuric acid which is corrosive and poses problems in terms of storage hazards,        bentonite which must be used in large quantities (of the order of 10%) which reduce the proportion of active ingredient accordingly,        or cellulose derivatives such as carboxymethyl cellulose (CMC) which are relatively expensive.        
Document DE 40 40 771 describes fertilizer particles consisting of a core comprising an organic material of natural origin consisting of wood and grape cake mixed with natural hydrosoluble polymers of the pectin or alginate type. The core is then encapsulated in gelatine. The proposed product is not a granule obtained by wet granulation but is equivalent to particles obtained by encapsulation. In addition, all the polymers mentioned are anionic polymers and the Applicant has observed that they were either impossible to use in a granulation process or have to be used in quantities that reduce the proportion of the active ingredient.
Document EP-A-255 752 describes fertilizer granules coated in a composition based on 3-hydroxy-3-alkylpropionic acid used as an antimicrobial agent in the presence of polymers that are capable of accelerating or retarding dissolution of said antimicrobial agent. Given their function, the polymers are polymers that are not soluble in water and not used as a binder but are used in order to retard release of the antimicrobial agent to a greater or lesser extent.
Document US 2004/009878 describes fertilizer pellets that include, as a binder, synthetic organic polymers such as polyacrylamide. These polymers are anionic polymers with the same disadvantages as those mentioned above.
The present invention intends to overcome these deficiencies.